Optogenetics is a relatively new technology in the field of neuroscience that combines genetic targeting of specific neurons or proteins with optical technology. Optogenetics is based on the genetic transfection of specific cell types to express photosensitive proteins, whose spiking activities can then be precisely controlled by light pulses of specific wavelengths. These light-responsive proteins, called opsins, are used to selectively turn neurons on or off with specificity and precise temporal resolution. Neurons now may be controlled with optogenetic tools for fast, specific excitation or inhibition within systems as complex as freely moving mammals. Probes are used to take advantage of genetic targeting strategies to express light-sensitive proteins in genetically defined populations of neurons, allowing unambiguous identification of the neurons under investigation. By using light-sensitive probes, it is possible to control the activity of entire populations of potential presynaptic neurons and/or monitor the responses of populations of potential postsynaptic neurons. Optogenetic tools can provide new ways to establish causal relationships between brain activity and behavior in health and disease. However, while the exploration of the wiring diagram of neural networks is moving forward at an unprecedented scale and steady innovations in optogenetics may provide a toolset for identifying and manipulating circuit components, innovative approaches that enable low-cost, practical solutions for optogenetic tools are lacking.
The development of reliable chronic brain implants that can access the activity of large populations of individual neurons with high spatial and temporal resolution is ongoing. Several groups have developed larger-scale optoelectrodes to deliver optical stimulation light to deep brain structures while simultaneously recording neurons. However, light sources placed on the surface of brain or large fibers placed in the brain parenchyma a few hundred microns away from the recording sites inevitably require excessive power to illuminate the large area of the brain and in turn, activate many untargeted neurons. A complete multi-color optical stimulation and electrical recording system was demonstrated using diode-coupled optical fibers attached to commercial multi-shank silicon probes. However, the manual attachment of fibers glued to portable light sources on probe shanks can be highly variable and labor-intensive. Recently, a monolithically integrated optical waveguide in a multi-electrode array silicon probe, precisely delivering light in the proximity of recording sites was developed. But in that case, the waveguide was connected to an on-bench solid-state laser source through optical fibers. Direct assembly of light sources on the silicon probe back-end was also introduced, but the issue of potential device heating, which can cause thermal damage to the surrounding brain tissue during device operation, needs to be addressed. Providing light sources on the probe shank and/or using an optical fiber to transmit light from the probe back-end to the optical emission port on the probe can lead to undesirable heating of the target site, and possibly tissue damage. Further, a reliable coupling scheme should be optimized for efficient optical coupling between the light source and the waveguide. Thus, while the exploration of optoelectrodes is moving ahead with advances in MEMS, microelectronics and optics, innovative approaches that enable practical solutions for multiple wavelength optogenetic tools for precise neural circuit manipulation are lacking.